As apparatuses become commercially available for performing microprocessing, e.g., etching or film forming, on a target object by plasma action, capacitively coupled (parallel plate type) plasma processing apparatuses, inductively coupled plasma processing apparatuses, and microwave plasma processing apparatuses are commonly utilized. Among these, a parallel plate type plasma processing apparatus applies high frequency power to at least one of an upper electrode and a lower electrode facing each other, to generate electric field energy, thereby exciting a gas to generate plasma, which processes a target object finely.
According to the recent need for miniaturization, it is inevitable to supply relatively high frequency power of, e.g., 100 MHz, to generate high density plasma. As the frequency of power supplied becomes higher, a high frequency current flows along the plasma-side surface of the electrode from its end portion to its central portion due to the skin effect. Such effect causes the electric field strength to be higher at the central portion of the electrode rather than at the end portion of the electrode. Accordingly, the electric field energy consumed for the generation of plasma at the central portion of the electrode is higher than that at the end portion of the electrode, and thus ionization or dissociation of a gas is further accelerated at the central portion of the electrode than at the end portion of the electrode. As a consequence, an electron density Ne at the central portion is higher than that Ne at the end portion. Because a resistivity of the plasma decreases at the central portion of the electrode with a higher electron density Ne, a current with a high frequency (electromagnetic wave) also focuses on the central portion in the facing electrode, thus leading to further nonuniformity of the plasma density.
Accordingly, it has been suggested to bury a dielectric material, e.g., ceramics, in the electrode near the central portion of the plasma-side surface (see, e.g., Japanese Patent Application Publication No. 2004-363552).
It has also been suggested to ensure higher uniformity of a plasma that the dielectric material be formed in a tapered shape or the dielectric material be made thinner in thickness as going from its central portion toward its periphery. FIG. 16 depicts a simulation result of an electric field strength distribution for four different constructions A to D of an upper electrode in a parallel plate type plasma processing apparatus. The construction A of the upper electrode 900 includes a base 905 made of a metal, e.g., aluminum (Al) and an insulation layer 910 made of alumina (Al2O3) or yttria (Y2O3) sprayed on the plasma-side surface of the base 905. The construction B of the upper electrode 900 further includes a columnar shaped dielectric material 915 with a dielectric constant ∈ of 10, a diameter of 240 mm, and a thickness of 10 mm, buried in the center of the base 910 in addition to the base 905 and the insulation layer 910. The construction C of the upper electrode 900 includes a tapered dielectric material 915 which is 10 mm thick at its central portion and 3 mm thick at its end portion. The construction D of the upper electrode 900 has a stepped dielectric material 915 that includes a first step with a diameter of 80 mm, a second step with a diameter of 160 mm, and a third step with a diameter of 240 mm. As a result, in a case where there is no dielectric material as shown in “A” of FIG. 16, the electric field strength was higher at the central portion of the electrode than that at the end portion of the electrode. This will be described with reference to FIG. 17A. Assuming that electric field strength distribution is E/Emax when the maximum electric field strength is Emax under each condition, it can be seen that the electric field strength distribution E/Emax at the plasma-side of the electrode 900 becomes dense at the central portion owing to a high frequency current flowing from the end portion of the electrode 900 to the central portion of that.
On the other hand, in a case where the columnar shaped dielectric material 915 shown in “B” of FIG. 16, the electric field strength distribution E/Emax was lowered at the bottom portion of the dielectric material. Referring to FIG. 17B, the capacitance component C of the dielectric material 915 and a sheath capacitance component (not shown) function as a voltage divider and the electric field strength distribution E/Emax is lowered at the central portion of the electrode 900. And, there occurs nonuniformity in electric field strength distribution E/Emax at the end portion of the dielectric material 915.
In a case where a tapered dielectric material 915 is provided as shown in “C” of FIG. 16, there was an improvement in uniformity of electric field strength distribution E/Emax made from the end portion of the electrode toward the central portion of the electrode. Referring to FIG. 17C, it is considered that since the capacitance component was higher at the end portion of the dielectric material 915 than at the central portion of that, the electric field strength distribution E/Emax was not excessively lowered at the end portion of the dielectric material 915 compared to a case where a flat type dielectric material 915 was provided and this allowed a uniform electric field strength distribution.
In a case where there is provided a dielectric material 915 having steps as shown in “D” of FIG. 16, there occurred steps in electric field strength distribution E/Emax as compared to the case where a tapered dielectric material 915 is provided as shown in “C” of FIG. 16. However, this case allowed a more uniform electric field strength distribution than the case where the columnar shaped dielectric material 915 is provided as shown in “B” of FIG. 16. The simulation result showed that the case, where a tapered dielectric material is provided, exhibited the most uniform electric field strength distribution E/Emax and thus this case allowed plasma to be generated most uniformly.
However, it suffers from the following problem to bury the tapered dielectric material 915 in the base 905. An additive or a screw is used to join the dielectric material 915 with the base 905. Since the base 905 is formed of a metal, e.g., aluminum and the dielectric material 915 is formed of ceramics, there occurs a difference in linear heat expansion. In consideration of this, there is a need for providing a proper gap between the members.
If the dielectric material 915 has a tapered shape, the dimensional accuracy is deteriorated at the tapered portion due to a lack of machining accuracy. This results in stress concentration due to a difference in heat expansion. The stress concentration is also caused by a difference in thermal conductivity due to a discrepancy in dimensional tolerance at the mating interface or a discrepancy in thickness of the dielectric material. An adhesive is peeled off from the mating interface due to the stress concentration. Since the difference in thermal expansion coefficient makes it difficult to manage the gap due to a difference in heat expansion, the peeled adhesive escapes from the gap, which causes a contamination in the chamber. Further, among the insulation layer 910 sprayed on the surface of the dielectric material 915 formed of ceramic or the like and the insulation layer 910 sprayed on the surface of the base 905 formed of aluminum or the like, it is likely for the insulation layer sprayed on the dielectric material formed of ceramic or the like to be peeled off due to a difference in adhesive strength. As a result, a contamination in the chamber is also caused by peeling of the material sprayed on the dielectric material 915.